Every version of XCode I've used has required the OS it is installed to be the latest one. 3.2 required Snow Leopard, 3.1 required Leopard, and so on and so for

Every version of XCode I've used has required the OS it is installed to be the latest one. 3.2 required Snow Leopard, 3.1 required Leopard, and so on and so forth. 4 might not require Lion but you can guarantee that 4.1 definitely will.